You can keep in mind that treants won't switch targets if you're just dotting up other targets so even on a multitarget fight they aren't necessarily a loss. I was using them on h magaera and they seemed fine even with the adds. Also even if they lag when switching targets as long as the target they are on doesn't die its also not really a loss. They are definitely situational but they shouldn't be written off. I didn't test it but I don't think they switch targets if you're using hurricane either. As slippys numbers show (don't remember where now) but I think they are theoretically better than incarnation on up to four targets unless target switching is causing them to lose casts.
Hopefully blizzard fixes target switching and losing casts with too much haste/targets dying, but they do have uses and can help smooth out your damage instead of simply having burst with incarnation. Gonna keep playing with them in the coming week to see how they fare on various fights. Also would be nice if they got damage bonuses from jinrok pools/ji kun feed young but its nice to have a talent choice and its easy enough to switch to incarnation when treants aren't cutting it. Its not like you need a different reforge to switch between them especially if you're using the haste reforge.